In a conventional conditional access system, data is transmitted by a transmitter to a plurality of receivers. The transmission can occur in many forms, for instance using terrestrial, satellite, or cable transmission. Usually, data is broadcast (i.e. the data is transmitted once for receipt by all eligible receivers). Access to the data is conditional, for instance depending on whether or not a subscription fee has been paid for a specific receiver. Such conditional access to the data services is realized by encrypting the data (usually the encryption occurs in the transmitter) under control of an authorization key and by transmitting the encrypted data to the receivers. Furthermore, the decryption keys necessary for the decryption of the data are encrypted themselves and transmitted to the receivers. Usually, symmetrical encryption techniques are used, where the encryption and decryption keys are the same. Only those receivers that are entitled to the data are able to decrypt the decryption key using a first decryptor. The receivers can then decrypt the data using a second decryptor for decrypting the data under control of the authorization key. Normally the encryption/decryption of the authorization key occurs in a secure environment. To this end, these functions are usually executed on a smart-card in or connected to the receiver. In the system describe above, the authorization key is used to directly control the encryption/decryption of the data stream.
It is also known from prior art systems to add one or more security layers to ensure that a malicious user does not retrieve the authorization key sent from the second decryptor to the first decryptor and supplies the key to data decryptors of other receivers. In such systems, the key used for encrypting/decrypting the data is changed frequently (e.g. once every second). This key is usually referred to as the content key. The content key itself is also transmitted (usually broadcast) to all receivers in an encrypted form (referred to as control word), using the authorization key. In this scenario, the authorization key directly controls the decryption of the control word, and indirectly the decryption of the data. The decryption of the control word also takes place in the secure module of the receiver.
It will be appreciated that it is important that the authorization can not easily be retrieved by malicious users. To that end, each receiver is equipped with a fixed device key incorporated in the secure module. The transmitter encrypts the authorization key separately for each receiver under control of the unique fixed key associated with the receiver. The transmitter sends the encrypted authorization key separately to each receiver. This requires a lot of messages and a high bandwidth in the system. This implies that the authorization key is only exceptionally updated, e.g. once a year. If an authorization key has been illegally retrieved, this usually does not result in updating the authorization key as long as the number of receivers illegally using the broken key is relatively low.